Unintended Parametric Ejection of Ions from an Ion Cyclotron Resonance Trap by Two-Electrode Axialization

Abstract

Azimuthal quadrupolar excitation is a commonly used technique in the field of ion cyclotron resonance mass spectrometry, in particular in combination with buffer-gas collisions to achieve axialization of the stored ions. If the quadrupolar excitation is applied with only one phase to a set of two opposing ring segments (rather than the "regular" method where two sets of electrodes are addressed with opposite polarities), parametric resonance effects at the frequencies 2nu(z) and nu(p) = nu(+) - nu(-) can lead to unintended ejection of ions from the trap. These parametric resonances have been revisited both theoretically and experimentally: multipole components of different azimuthal excitation schemes are derived by a simple vector representation of the excitation signal applied to the ring segments. Thus, parametric contributions can be easily identified, as demonstrated for the two-electrode and the four-electrode quadrupolar excitation schemes as well as further examples. In addition, the effect of the single-phase two-electrode quadrupolar excitation is demonstrated for storage and axialization of cluster ions.

Dipolar and quadrupolar detection using an FT-ICR MS setup at the MPIK Heidelberg

“…A regular quadrupolar detection would make use of all four segments and would apply alternately plus and minus signs while summing all signals. In order to discuss the restriction of the present detection scheme, the above mentioned theorem of reciprocity can be invoked again: Just as a single-phase, two-electrode configuration can be used for a quadrupolar excitation at (ν + + ν − ) [26], it allows the detection of the ion image-charge signal at that frequency. Thus, this configuration is used for excitation and then detection at (ν + + ν − ) (see Section 3).…”

“…The (ν + + ν − ) signal also has an higher amplitude than the (ν + − ν − ) signal which may be called a radial parametric resonance frequency [28,30]. This signal appears due to the realization of quadrupolar detection by use of only one set of electrodes instead of the ideal configuration of the difference signal from two sets of opposite electrodes [26]. …”